Instantaneous ultrasonic echo measurement of bladder volume with a limited number of ultrasound beams

ABSTRACT

An apparatus and methods to quantify the volume of urine in a human bladder with a limited number of acoustic beams is disclosed. In a first version a plurality of narrow ultrasound beams is transmitted in different directions towards the bladder. Returning echoes are converted to digital form and stored in memory. A volume display on the apparatus allows to define the optimal apposition of the transducer assembly. Signal processing software automatically determines the bladder Depth D and Height H and computes the volume of urine. In a second version, a single wide angle ultrasound beam transducer transmits ultrasounds signals at a fundamental frequency to quantify the urine volume. Return signals originating from a depth beyond the usual position of the posterior wall depth of a filled bladder are analyzed for presence of higher harmonic signals, which in turn are related to the presence or absence of urine.

PRIORITY CLAIM

This application is a continuation of application Ser. No. 11/010,539U.S. Pat. No. 7,749,165, which claims priority to PCT/EP2003/007807filed Jul. 1, 2003, which application claims priority to UK PatentApplication No. GB 2 391 625A filed Aug. 9, 2002. Each of the foregoingapplications is incorporated by reference as if fully set forth herein.

The present invention relates to methods and apparatus for themeasurement of volume of a fluid filled cavity in a human or animalbody, such as a bladder, using ultrasound techniques.

TECHNICAL FIELD

This invention concerns an apparatus which, in a first version, with alimited number of fixed ultrasound transducers with narrow sound beamsoriented in well defined directions, automatically determines the volumeof the human bladder without assumption of any geometrical bladdershape, where volume is calculated by (Height×Depth×K) and theempirically measured K factor varies with bladder filling degree, whichin turn is indicated by the number of ultrasonic beams that interceptthe filled bladder. In this first version, standard echographictechnique is used where short ultrasound pulses are transmitted atfundamental frequency and the echo travel time is used to calculatedistance.

In a second version, with a wide ultrasound beam, pulses are transmittedat fundamental frequency. Due to the wide sound beam this beamencompasses a large part of the volume of a possibly filled bladder.Echo signals from a large distance W, where W is the average distancefrom the transducer in dorsal direction to a point beyond the posteriorwall of an average filled bladder, are analyzed for higher harmoniccontents. Non-linear behavior will increase with depth and particularlybe stimulated by presence of urine. Attenuation of returned echo signalsfrom a large distance will be considerably less in the presence ofurine. A combination of these two effects will favor presence of higherharmonics as compared to the presence of the fundamental frequency inthe return signal. With this information urine quantity or a criticalurine filling level of the bladder can be established.

In a third version a combination of a narrow ultrasound beam fordetection of the posterior bladder wall distance W with the wideacoustic beam approach for subsequent measurement of urine filling ofthe bladder is described.

BACKGROUND OF THE INVENTION

It is well known that bladder dysfunction is associated with a number ofclinical conditions requiring treatment. In many of these cases it isimportant to accurately determine the volume of the bladder. Under otherconditions such as post-operative recovery, where there is temporaryloss of bladder sensation and/or loss of the normal voiding mechanismtoo much distention of the bladder has to be avoided. Under thoseconditions voiding by catheter introduction is carried out. However,serious disadvantages to unnecessary catheterization range from theuncomfortable situation for the patient to serious possibilities ofinfection. Thus, a non-invasive quick measurement of bladder volume,with the patient usually in the supine position, is indicated. Sometimesthe accurate determination of volume is indicated; sometimes however anindication is sufficient. Questions that may be asked are for instance:after voiding: “is there still too much urine left?”; or after surgery“is the bladder filling above a certain level so that voiding isnecessary?”

Non-invasive procedures for bladder volume estimation are known, but areeither unreliable or expensive or have some other significantdisadvantages. Palpation and auscultatory percussion are known to beunreliable, while radiography and dye-excretion techniques are known tobe similarly inaccurate. For assessing bladder volume, catheterizationremains the “gold standard”. However, it is invasive, painful and mightproduce traumas or infections.

Subject

The described technique concerns measurement of urine volume in thehuman bladder with the use of pulsed ultrasound with a limited number ofultrasound transducers.

In a first version a limited number of transducers are mounted in atransducer assembly. The assembly is positioned non-invasively at thebody skin over the position of the bladder with the patient in a supineposition. For acoustic contact a coupling gel may be used. Eachultrasound transducer in the assembly transmits and receives theultrasound signal in a narrow beam through the contact plane. During themeasurement the transducers are used in a certain succession. Alltransducers have been mounted in the assembly such that in transmissionand reception successively the beams penetrate the area of the bladderin approximately the sagittal cross sectional plane. The sagittal planeis here defined as ANTERO-POSTERIOR plane of the body. One transducerbeam direction is dorsal with in addition at least one transducer beamin the dorsal-caudal and one transducer beam in the dorsal-cranialdirection. The volume is calculated on the basis of two bladdermeasurements defined in the sagittal plane as Depth (D) and Height (H).These measurements are derived on the basis of echo travel time fromechoes originating at the anterior and posterior bladder wall. Depth isin principle a measurement in dorsal direction. Height is a measurementapproximately in the cranial direction. The volume is calculateddepending on the specific, filling dependent, measurement configurationfollowing the formula D×H×K. Where K is an empirically measured, fillingconfiguration dependant, correction factor. Beam directions and examplesfor D and H are illustrated in FIGS. 1 and 2.

In a second version of the described technique a single wide beamultrasound transducer is positioned non-invasively at the body skin overthe location of the bladder. The wide beam can be created by the curvedsurface of the transducer or by a flat acoustically active surface offor instance a disk shaped transducer supplied with a curved lens.Ultrasonic signals are transmitted and received in the wide, cone like,ultrasound beam and propagation is approximately spherical. Similar tothe above described method a pulsed echo signal is transmitted atfundamental ultrasonic frequency. In this second version of thedescribed technique echo data are analyzed as originating from adistance beyond the average position of the posterior (filled) bladderwall. The received echo signal will contain information over almost theentire bladder as encompassed by the wide ultrasound beam. Due tonon-linearity, higher harmonic components will build up duringpropagation and thus be reflected in the returning echo.

Compared to propagation through normal tissue, the presence of higherharmonics in the signal is greatly stimulated when propagating throughurine. Analyses of presence of higher harmonic components in relation tothe fundamental frequency is used for indication of presence of urine inthe bladder. Neutralizing patient variation as to obesity etc can alsobe accomplished by comparing echo signals received from sequentiallytransmitted pulses at low transmit power (linear propagation only) andpulse transmission at high power (enhancing non-linearity).

State of the Art

Non-invasive bladder volume measurement techniques with ultrasoundechography have been described in the art. In principle, echographymeasures distance based on echo travel time. Early echo techniques diduse a single ultrasound transducer and echo presentation was recorded asecho amplitude versus depth. West, K A., “Sonocystography: A method formeasuring residual urine,” Scand J Urol Nephrol 1: pp 68-70, 1967describes the subsequent use of some discrete beam directions. He doesnot have a separate transducer for each beam direction. His method isonly qualitative, not instantaneous, and based on distance measurementto the dorsal posterior bladder wall. His method is not adjusted tospecific, filling dependent, measuring configurations. A relationbetween the difference in echo travel time between echoes from theposterior an anterior bladder wall and the independently measuredbladder volume has been reported by Holmes, J H: “Ultrasonic studies ofthe bladder”, J. Urology, Vol 97, pp. 654-663. His described volumemeasurement method is exclusively based on bladder depth measurement.Since the bladder changes in shape when filling, a single distancemeasurement is not precise enough to predict the entire bladder volume.No filling dependent measurement configuration is used.

Diagnostic ultrasound is today well known for real-time cross-sectionalimaging of human organs. For cross-sectional imaging the sound beam hasto be swept electronically or mechanically through the cross section tobe imaged. Echoes are presented as intensity modulated dot on thedisplay. The instruments are costly and require a skilled operator.Volume is sometimes calculated based on bladder contours obtained in twoorthogonal planes with a geometric assumption of bladder shape. For3-dimensional or volumetric echography the sound beam has to be sweptthrough the entire organ. This further increases complexity, acquisitiontime of the data, and costs of the instrument.

HAKENBERG ET AL “THE ESTIMATION OF BLADDER VOLUME BY SONOCYSTOGRAPHY”, JUrol, Vol 130, pp 249-251, have reported a simple method that is basedon measuring the diameters obtained in a cross sectional image in themidline sagittal bladder plane only. The bladder volume has been relatedto bladder Height and Depth as follows: Volume is Height×Depth×6.6 ml.This formula showed a good correlation coefficient (r=0.942) with arelatively large average error of 30.1%. For this approach atwo-dimensional imaging apparatus was required. The used apparatus iscomplex and is different from the method described in this application.It does not use a single wide beam transducer or a limited number offixed transducers in an assembly or a combination of this.

An ultrasound apparatus for determining the bladder volume is shown inU.S. Pat. No. 4,926,871 in the name of Dipankar Ganguly et al. In thistext, a number of possibilities are mentioned, amongst which a scan headembodiment referred to as a sparse linear array with transducers mountedat predetermined angles with sound beams pointing towards the sameposition. The volume is calculated according to a geometric model. Inthe claims an apparatus is described, involving an automatic calculationof bladder volume from ultrasound measurements in a first and secondplane, which are substantially orthogonal to each other. Sound beams aredeflected by a stepper motor. It requires a skilled operator tomanipulate the scan head in a particular way to obtain the ultrasoundmeasurements. For the volume calculation method described in thisapplication no use is made of any geometrical model of the bladder,whereas only a limited number of sound beams approximately in thesagittal plane, or a single wide beam is used.

Volume measurement based on echographic sampling of the bladder with ahand guided transducer mounted in a panthograph has been described byKruczkowski et al: “A non-invasive ultrasonic system to determineresidual bladder volume”, IEEE Eng in Medicine & Biology Soc 10TH AnnConf, pp 1623-1624. The sampling covers the entire bladder, follows agiven pattern and is not limited to a single or two cross sections ofthe bladder. For the calculation he needs data from many beamdirections. The acquisition procedure is time consuming and thus noinstantaneous volume measurement results. The method described in thisapplication is based on use of a single, wide beam or the use of alimited number of mutually fixed sound beams directions withinstantaneous volume indication.

The hand steered transducer guiding for recording of echo data from thebladder has subsequently gained in acquisition speed by introduction ofconstructions whereby the transducer, and thus the beam, wasmechanically swept. This nevertheless still requires an acquisition timeequivalent to full acquisition procedure and thus does not yield aninstantaneous display of volume. No instantaneous feedback on optimalpositioning is thus available. An example of such methods is theBLADDERSCAN. In the Bladderscan Technology (registered trademark ofDiagnostic Ultrasound Corporation) bladder volume is measured byinterrogating a three-dimensional region containing the bladder and thenperforming image detection on the ultrasound signals returned from theregion insonated. The three dimensional scan is achieved by performingtwelve planar scans rotated by mechanically sweeping a transducerthrough a 97 degree arc in steps of 1.9 degrees. The three dimensionalscanning requirement makes this instrument complex. It can not becompared with the simple approach described in this application.

Yet another ultrasound method “System for estimating bladder volume” isdescribed by Ganguly et al in U.S. Pat. No. 5,964,710 dated Oct. 12,1999. This method is based on bladder wall contour detection withechographically obtained data in a plurality of planes which subdividethe bladder. In each single plane of the plurality of planes a number ofN transducers are positioned on a line to produce N ultrasound beams tomeasure at N positions the distance from front to back wall in theselected plan. From this the surface is derived. This procedure isrepeated in the other planes as well. The volume is calculated from theweighted sum of the plurality of planes. In Ganguly's method the entireborder of the bladder is echographically sampled in 3 dimensions. Hismethod differs strongly from the method described in this applicationwhereby only a single wide beam is used or a limited number of mutuallyfixed sound directions are used in approximately a sagittal plane with afilling dependent measurement configuration.

U.S. Pat. No. 6,359,190 describes a device for measuring the volume of abody cavity, such as a bladder or rectum, using ultrasound. The deviceis strapped to the body or incorporated into a garment such as a nappyor trainer pant. The device includes several transducers each aimed at adifferent region of the subject's bladder (a) to ensure that at leastone ultrasound beam crosses the bladder despite variations in the waythat the device has been positioned on the body, and (b) to enable thetransducer with the strongest signal output to be used. An alarm signalmay be output when the bladder reaches a predetermined threshold volume.

An important parameter for assessing bladder volume if this volume hasto be derived from a limited number of beams or planes is the knowledgeof bladder shape and position which can drastically vary with age,gender, filling degree and disease. In the adult patient the emptybladder has the shape of a triangular prism and is located behind thepubis. When it is progressively filled, there is first a distention ofthe bladder depth followed by an expansion of the bladder height. Thebladder shape is complex and can not be represented by a singlegeometrical formula such as ellipsoid, sphere etc. This explains thelarge error that several studies obtained when a single geometric modelwas used. However there exists a correlation between the bladder heightand the bladder widening with progressive filling.

In the first approach of the present invention an instrument isdescribed which allows assessment of bladder volume by using only a fewultrasound beams appropriately oriented in approximately the sagittalplane. The narrow sound beams in principle diverge relative to eachother. This allows covering a wide range of filling degrees of thebladder, from almost empty, when the bladder is located behind thepubis, to a full bladder that causes a substantial bladder height (SeeFIGS. 1 and 2). From each beam can be established, by detection of theposterior bladder wall echo, if this beam does pass a filled bladder.From the knowledge of all beams that do pass the filled bladder theappropriate filling or measurement configuration follows. The acousticbeams are positioned in such a way that the Depth D and Height H of thebladder can be estimated for the specific measurement configuration. Thevolume of urine is then computed from an empirical formula D×H×K thatdoes not depend on any geometric model. K is a known, empiricallyestablished correction factor which is specific for each measurementconfiguration and has been established by calibrated bladdermeasurements on a prior series of patients. The accuracy of the firstapproach is thus based on an a prior known correction factor which isrelated to a specific filling degree, which in turn depends on thenumber of beams that intercept the filled bladder.

A second version of the instrument is based on the measurement of thepresence of higher harmonics in the echo signal. For this approach theecho signal from a depth greater than the distance from the transducerto the posterior bladder wall must be analyzed. For a filled bladder inadults in a supine position, this depth W would be approximately 12 cm.

It is known that when sound pulses are transmitted at a fundamentalfrequency, higher harmonics of this fundamental frequency may be presentin the received echographic signal. Non-linear distortion increases withdistance, insonifying ultrasound energy and frequency. Attenuationdiminishes the ultrasound amplitude with increasing propagation distanceand reduces the higher harmonic energy. Since attenuation of theultrasound signal in urine is low compared to tissue and non-lineardistortion in urine is large compared to tissue it results that urine isvery different from tissue in its ability to generate higher harmonics.We have measured the presence of higher harmonics in the echo signalfrom 12 cm depth when the bladder was filled. With an empty bladder theechoes obtained from the same depth did not contain higher harmonics.

The interest of higher harmonic signals in the ultrasound techniquestems from echo contrast technology. Echo contrast material containscoated gas containing micro bubbles suspended in a fluid. These bubblescan create higher harmonic components in the echo signal due tonon-linearity. This is used to indicate presence of contrast on thediagnostic image. A wide variety of pulse techniques is used tostimulate echographic visibility of contrast. These include multi pulseprocedures, multi frequency procedures, power Doppler imaging, pulsecoding, pulse inversion and other imaging methods. A survey isdocumented in “Ultrasound Contrast Agents” ISBN 1-85317-858-4 chapter 3“Contrast-specific imaging methods” by de Jong et al. With a singletransducer with wide sound beam, such as results with a curved acousticelement or a flat, disk shaped transducer plus curved lens, thepropagating sound beam would encompass the entire bladder. Thetransducer must be designed to optimally transmit the fundamentalultrasound frequency and at the same time be capable to receivefundamental and higher harmonic echo signals. Broadband piezo-electricceramic transducers have been described as well as combinationtransducers using ceramic in transmission and PVDF material inreception. In transmission a single or multi pulse procedure can befollowed. If the returned echo signal with such a method would, inrelation to the fundamental echo signal, be analyzed for the presence ofhigher harmonics, the presence of a certain level of bladder filling orthe volume of urine can be established.

EP 0271214 describes an ultrasonic device for monitoring the volume offluid in the human bladder by using reflected ultrasound signals todetermine not only the position of the bladder back wall but also energyreturned from the bladder back wall. EP '214 proposes that after bladderfilling to approximately 60% capacity, the distance between the backwall and the front wall of the bladder stops increasing. However,additional reverberation in the back wall provides an increase in energyin the reflected signal which can be used to determine further increasesin bladder volume.

DESCRIPTION OF THE DRAWINGS

FIG. 1 Illustrates a sagittal (anteroposterior) cross sectional plane ofa patient in supine position where a transducer assembly 1 withtransducers A, B, C, D and E, is positioned on the abdominal wall justabove the Symphysis Pubis 2 and the ultrasound beams are indicated tocross the area of the partially filled bladder 3. From the transducerassembly, the sound beam A intercepts the bladder area in dorso-caudaldirection, soundbeam B intercepts the bladder in dorsal direction andsound beams C, D, and E respectively in dorso-cranial direction. In FIG.1 the patient's leg is indicated by 4.

FIG. 2 Illustrates various bladder filling stages from an almost emptybladder to a strongly filled bladder and the corresponding measurementconfigurations. Depth D and Height H have been defined for each fillingsituation as indicated and are calculated from detected bladder wallechoes taking the specific measurement configuration into account. Foreach measurement configuration a specific Depth D and Height H isdefined.

FIG. 3. Illustrates, by way of example for a transducer assembly withfive transducers (here only A and D, necessary for calculation of H areshown), the calculation of Height H (5) in the measurement configurationwhen bladder posterior wall echoes are detected originating from soundbeam A, B, C and D. This is the “filled bladder” measurementconfiguration shown in FIG. 2. Apparently no posterior wall echoes aredetected in sound beam E because the bladder filling is not yet in astrongly filled stage and thus beam E does not intercept the bladder.Depth D is derived from beam B (not shown in FIG. 3).

FIG. 4 Represents a flow chart of the actions of the principal hardwarecomponents. In this block diagram a “useful” transducer signal occurswhen bladder wall echoes are detectable in its sound beam.

FIG. 5. Illustrates a top view of five disk shaped transducers in apossible transducer assembly. The distance between transducers B, D andC, A, E and their positioning is such that all sound beams can beassumed to be in approximately a sagittal cross section through thebladder. Yet another transducer assembly with 4 transducers in a row isalso illustrated.

FIG. 6. Illustrates a cross sectional view showing in the lengthdirection a possible transducer and related sound beam orientation whenfive single transducers are used.

FIG. 7 Illustrates the sagittal cross sectional plane with a single widebeam transducer non-invasively positioned on the abdominal skin surfaceover the filled bladder 3. Echo signal is received from a range at depthW.

FIG. 8. Is a flow chart illustrating the principal steps taken by thebladder volume measurement instrument based on a single ultrasound widebeam where detection of presence of higher harmonics in the receivedsignal from a give depth range is used to measure volume. Two differenttransmit levels are used to enhance the bladder effect and eliminatepatient variation.

FIG. 9. Illustrates the measured received scattered power in thefundamental frequency to and the higher harmonic frequencies 2f₀ and 3f₀in a situation with an empty versus a filled bladder.

FIG. 10 shows two possible transmit pulse sequences to enhance thedifference between linear and non-linear sound propagation.

FIG. 11 Illustrates a possible look-up table based on prior calibratedpatient bladder volume measurements relating presence of harmonic powerin the received echo signal versus volume.

DETAILED DESCRIPTION OF THE FIRST METHOD

The first method describes a simple device that allows the assessment ofbladder 23 volume, using only a few beams appropriately oriented. Underthe assumption that there exists a correlation between the bladderheight and width, a simple approach has been developed. It consists of alimited number of acoustic beams positioned in such a way that the depthD and the height H of the bladder could be estimated in approximately asingle sagittal plane. The volume of urine is then computed from anempirical formula that does not assume any geometric model.

In operation of the apparatus of the present invention, the transducerassembly 1 is placed on the abdomen of the patient in the supineposition, just above the symphysis pubis 2. We are presenting aparticular configuration of the assembly 1. Nevertheless, variousconfigurations can be derived from this model and several modificationscould be achieved (number of transducers, position, orientation, etc. .. . ) without departing from the initial ideas. The device proposed asan example is composed of five disc shaped transducers A, B, C, D and E(focused or non-focused) positioned in the assembly at predetermineddistance from each other (FIG. 5, top panel) and oriented atpredetermined angles Ø_(A), Ø_(B), Ø_(C), Ø_(D), and Ø_(E) (FIG. 6).Referring to FIG. 5 (top panel), it appears that the transducers A, B,C, D and E are oriented in two different planes. The distance betweenthese two planes is small compared to the bladder 3 size and thus we canassume that the information received from each transducer represent thecharacteristics of approximately a single sagittal or anteroposteriorplane. The orientation of each beam has been determined from theknowledge of the bladder 3 position and shape when it is filling up asmeasured in a patient series. The first beam of the transducer assembly1 (soundbeam from transducer A) is oriented in such a way that itreaches the bottom of the bladder, passing just above the symphysispubis 2. The remaining beams are positioned for successivelyintercepting the bladder 3 when it expands with increasing fillingdegree.

Computation of the Depth D and Height 5: Depending on the number ofbeams that are intercepting the bladder 3 and on the geometricalconfiguration of the transducer assembly (1), the distances H and D aredetermined by different mathematical procedures. For most measurementconfigurations the depth D of the bladder is determined by the distancebetween echoes derived from front and back wall of the bladder estimatedfrom Transducer B.

The Height H (5) calculation in the specific measurement configuration(here we selected as an example the “filled bladder” configuration ofFIG. 2) when posterior bladder wall echoes are detected in signalsobtained in beam A, B, C, and D, but not in beam E is illustrated inFIG. 3. For the other filling geometries the height is calculated in acorresponding way. The mathematical procedure is as follows:cos Ø_(A) =[AA2]/[AA1]=>[AA2]=cos Ø_(A) ·[AA1]  (1)sin Ø_(A) =[A1A2]/[AA1]=>[A1A2]=sin Ø_(A) ·[AA1]  (2)cos Ø_(D) =[DD2]/[DD1]=>[DD2]=cos Ø_(D) ·[DD1]  (3)cos Ø_(A) =[D1D2]/[DD1]=>[D1D2]=sin Ø_(D) ·[DD1]  (4)cos Ø_(A) ··=[AA2]/[AA1]=>[AA2]=cos Ø_(A) ·[AA1]  (5)cos Ø_(A) ··=[AA2]/[AA1]=>[AA2]=cos Ø_(A) ·[AA1]  (5)ID1=[D1D2]+[A1A2]+[AD]  (6)=>Height=[A1D1]=√{square root over ([A1I] ² +[ID1]²)}  (7)

Volume computation: The volume of urine is correlated to the bladderdiameter (Height 27 and Depth 26) by the empirical formulae:Height*Depth*K

where K is a correction factor. Depending on the number of beams thatallow the determination of the bladder dimensions (from 1 to 5) andothers parameters such as the age, the gender, the correction factor isdifferent. For a given situation (parameters other than number of beamare fixed), the correction factors KL, K2, K3, K4 and K5 are optimizedusing linear regression analysis.

The process executed by the hardware is illustrated in the flow chart ofFIG. 4.

After positioning the transducer assembly correctly over the bladderarea the measurement procedure is started by pressing the start buttonwhich during the (short) measurement procedure remains depressed.Subsequently the transducers are activated for transmission ofultrasound pulses and reception of echoes and possible detection ofbladder wall echoes in a specific order. Thereafter it is established,when a clear posterior bladder wall echo is detected, which ultrasoundbeams, this we call here the beams of “useful” transducers, penetratethe filled bladder. From this, the filling situation or measurementgeometry is established. As a result the proper correction factor can beselected. After calculation of the volume the value is stored in memoryand displayed. During the measurement procedure the transducer assemblyis slightly moved and memory data are refreshed if a larger volume ismeasured. The highest value will correspond with the correct bladdervolume. This is displayed.

In a general aspect, therefore, the apparatus may use beam informationcomprising at least: angle of incidence (known from the transducermounting angle), spatial position (known from the transducer position inthe array) and echo travel time (deduced from the reflected beam). Otherbeam parameters or information from reflected beams may also be used inaccordance with known ultrasound techniques, such as frequency, pulserate etc.

For determining body cavity and height, the apparatus may select onlybeams corresponding to those that have intercepted the fluid filled bodycavity.

The arrangements described in connection with FIGS. 1 to 6 illustrateuse of five transducers. This configuration was selected in order toachieve a selected degree of accuracy of measurement over a completeexpected range of total volumes in a human adult. In the preferredconfiguration, accuracy of measurement of the order of 100 ml over arange encompassing a bladder fill level from 0 to approximately 800 mlhas been exhibited. It will be understood that a smaller number oftransducers could be used when either the desired measurement accuracycan be reduced, or when the total fill range covered can be reduced.

For example, using just three transducers, it has been shown to bepossible to cover a fill range of 0 to approximately 500 ml with anaccuracy of 100 ml.

Similarly, four transducers has been shown to cover a range 0 toapproximately 700 ml, and two transducers, a range of 0 to approximately300 ml.

Such configurations can be used when it is only necessary to indicategross ranges of bladder filling, or to indicate a clinically importantthreshold fill level.

In other embodiments, the apparatus may be provided with an input devicesuch as a keypad or computer interface so that the user can enterpatient information, such as gender, weight and age. This informationcan then be used to ensure correct selection of an available correctionfactor, K, from a memory of the apparatus.

The apparatus may also be provided with means for inputting calibrationdata, such as absolute measurements of bladder fill level separatelydeduced from conventional measurements. These can be stored by theapparatus and used to optimise stored K values as part of an iterative,‘self-learning’ process. In other words, the apparatus may incorporatean algorithm for automatically adjusting predetermined correctionfactors stored therein based on calibration data entered into themachine for comparison with measurement data taken by the apparatus.

The apparatus may also comprise a means for indicating correctcaudal-cranial positioning of the transducer array on the body over thebladder. For example, in a normal measurement as suggested in figure, itis expected that at least transducers A, B and C will indicate a bladderpresent condition, whereas transducers D and E might, or might notindicate bladder present, according to the bladder fill level. In theevent that, for example, no signal is indicated by A, or by A and B, butsignal is indicated by D or D and E, then it can be deduced that thetransducer assembly is positioned too far in the cranial direction. Thiscould be indicated on the display of the device.

In summary, the described first method differs greatly from known otherapparatus:

1) The device is composed of a limited number of static single elementtransducers;

2) The arrangement of the transducer is not similar to the arrangementof a linear array;

3) The transducers are oriented towards the bladder with specific anglesallowing the estimation of the urine volume over a wide range ofvolumes;

4) The method for automatic volume computation does not assume anygeometrical model for the bladder shape;

5) It is valid for any bladder shape since the volume is computed withan empirical formula for various filling ranges;

6) It is not based only on the measurement of distances between thefront and back wall or area in different planes;

7) It uses an automatic detection of the bladder height and depthdepending on the number of beams that intercept the bladder;

8) It optimizes the correction factor depending on the degree of filling(or other factors, such as age, gender, weight, that may influence thecalculations);

9) The device includes a closed loop to easily find the optimalposition;

10) The optimal position corresponds to the largest volume computed;

11) The device works instantaneously.

DETAILED DESCRIPTION OF THE SECOND METHOD

The second version of the device is based on a different principle. Theapproach consists of using a single acoustic beam with a very wide widthsuch that it encloses approximately the entire volume of the bladderwhen it is filled up. Such a wide beam width can be obtained using asingle element transducer with a defocusing lens as drawn in FIG. 7 or acurved single element transducer.

The schematic principle of transducer positioning is illustrated in FIG.7. The sagittal cross section through the bladder is shown. The conelike shape of the acoustic beam allows to encompass approximately thefull bladder volume, and therefore any harmonic distortion detected inthe echo signal returning from a region beyond the posterior wall of thebladder around depth W, would correlate to the amount of fluid containedin the bladder.

It has been demonstrated that the propagation of ultrasound waves is anonlinear process. The nonlinear effects, which increase with higherintensities, have been predicted and demonstrated at frequencies andintensities used in the diagnostic range either in water or in humanbody (A Baker et al.: “Distortion and High-Frequency Generation Due toNon-Linear Propagation of Short Ultrasonic Pulses From A Plane CircularPiston”, J. Acoustic Soc Am 92(3), pp 1699-1705). The distortion is dueto slight non-linearities in sound propagation that gradually deform theshape of the propagating sound wave, and result in development ofharmonic frequencies which were not present in the transmitted waveclose to the transducer. This manifests itself in the frequency domainas the appearance of additional harmonic signals at integer multiples ofthe original frequency.

These effects occur most strongly when ultrasound waves propagate withinliquids with relatively low acoustic attenuation such as water, amnioticfluid or urine. Indeed, acoustic propagation in fluids gives rise toextreme nonlinear effects at diagnostic frequencies. Within softtissues, nonlinear processes also take place but are modified as aresult of the different acoustic characteristics of these tissues, mostnotably their high acoustic absorption. Indeed, water and amnioticfluids (urine) are significantly different from tissue.

It is known from literature (A C Baker: “Prediction Of Non-LinearPropagation In Water Due To Diagnostic Medical Ultrasound Equipment”,Phys Med Biol 1991 VOL 36, NO 11, PP 1457-1464; T Szabo et al.: “Effectsof Non-Linearity On The Estimation Of In-Situ Values Of Acoustic OutputParameters”, J Ultrasound Med 18:33-41, 1999; M Hamilton et al.:“Nonlinear Acoustics”, Academic Press) that the non-linearity of amedium is characterized by the coefficient of non-linearity β. Typicalvalues for P are 3. 6 for water, 4 for blood and 6.5 for fatty tissue.

In addition to being nonlinear, all the media have acoustical loss dueto absorption. The acoustical loss is described by the power law: A=AOFBwhere ao is constant and b ranges from 1 to 2 depending on the medium.For water, the rate of absorption of an ultrasound wave propagatingthrough it is quadratically related to the frequency (b=2). However, therate of energy loss due to absorption is considered small and most ofthe time the dissipation-less theory is applicable over short ranges.However, biological media have large rates of energy loss and thefrequency dependence has an exponential value of 1 to 1.5.

By considering both attenuation due to absorption loss andnon-linearity, the exchange of energy between the two processes iscomplicated, because attenuation diminishes the amplitude of thegenerated harmonic components with propagation distance whilenon-linearity builds up these harmonics. So, harmonic distortiongenerally tends to enrich the higher harmonic components at the expenseof the lower ones (energy transfer), while absorption damps out thehigher components more rapidly than the lower ones. It is thereforedifficult to reach a balance in which a given component loses as muchenergy by absorption as it gains from nonlinear distortion. Moreover,since the conditions for stability depend on the amplitude of the wave,which slowly decreases with propagation distance, the wave can never becompletely stable, only relatively so.

The balance between the nonlinear process and the attenuation process isgiven by the Goldberg number Γ (Szabo et al.), which represents ameasure of which process dominates. When Γ=1, nonlinear effects arecomparable to attenuation effects. If Γ is higher than 1, nonlinearprocesses dominate and when the Goldberg number is below 1, attenuationeffects take over. As indication, for acoustic pressures of 500 kPa andLMPA, at a transmit frequency of 3 MHz, the Goldberg number isrespectively 86.5 and 43.2 for water. It is only 2.8 and 1.4 forliver-like tissue respectively at these pressures. For both settings,the parameter shows that for water, non-linearity is up to thirty timesgreater than for tissue.

The approach used here is based on the “non-linearity/attenuation”characteristic in differentiating between fluid media and soft tissuemedia. As described above, a single element transducer is placed infront of the bladder. The transducer generates a wide acoustic beam thatis able to enclose the full bladder volume. Depending on the volume ofurine contained in the bladder (bladder filling) and thus crossed by theacoustic beam, the amount of harmonic distortion generated in the backof the bladder will change. A radio frequency (RF) backscattered signalmight be selected from a region of interest located preferably in thebackside of the bladder. The amount of energy comprised in the secondharmonic or higher harmonic components of the received RF echo signalcan be extracted and correlated to the amount of volume of urine thathas been encompassed by the acoustic beam. Since harmonic generation isdifferent in tissue than in fluids, only the volume of urine that hasbeen crossed by the acoustic beam would generate more harmonic energy.When the bladder is empty or below a certain volume level, no harmonicdistortion occurs, whereas maximal distortion will be obtained for afull volume.

FIG. 9 illustrates the principle of the invention. Top panel shows twosituations. The bladder is either empty (Panel A left side) or filled upwith urine (Panel A right side). At a certain distance beyond thebladder (around 12 cm from the transducer), a region of interest of 1.5cm width at depth W (see FIG. 7) is selected. Power spectracorresponding to echo signal recorded from the regions of interest aredisplayed in panel B.

The spectrum corresponding to the empty bladder (solid line) shows onlya fundamental component. The harmonic distortion is very weak so that noharmonic frequencies are generated. However, the echo signalcorresponding to the filled bladder situation (dashed line) demonstratesclear distortion where a second harmonic component with a significantenergy is generated. The third harmonic component can be also presentwith lesser energy depending on the urine volume that has been crossedby the acoustic beam.

FIG. 9 demonstrates that depending on the volume contained in thebladder that the acoustic beam has intersected, the amount of generatedsecond harmonic energy varies. When the acoustic beam crosses onlytissue or when the volume of urine is very small, harmonic distortion isthe lowest with no or very low harmonic energy. If the bladder is filledup or if the volume of urine is above a certain level (threshold),harmonics are generated. The generation of a harmonic component (secondand/or higher harmonics) can be used for volume measurement, or simplyas an indicator of filling of the bladder to a certain volume extent.The criterion can be such that if a certain amount of second harmonic(or higher harmonics) is generated in the echo signal, the device wouldindicate that the critical volume (or threshold) (say in adult patientsaround 450 ml) has been reached.

To avoid and eliminate any differences due to patient to patientvariations, a normalization procedure needs to be performed a priori.Such a normalization procedure might consist of recording a first signalat very low transmit acoustic power from the same region of interest asdescribed in the previous section. Such power would allow only linearpropagation of the ultrasonic waves and avoid any harmonic generation.The echo signal would therefore have undergone only attenuation effects.

In the following transmit-receive sequence, the transmit acoustic poweris increased with a certain factor (e) and a new recording is performedfrom the same region of interest. This measure with a much higheracoustic pressure is carried out to allow harmonic distortion to occurin the tissue. The echo signal in this case will undergo bothattenuation and distortion effects. The first echo signal (linear case)will be re-scaled by the factor that corresponded to the increase intransmit power (e), and then used as a reference signal. Consequently,each patient has his own reference hence eliminating any variations suchas obesity, INHOMOGENEITIES, etc.

A block diagram of a possible steps describing the second method isgiven in the flow chart of FIG. 8. The two transmitted signals might betransmitted with a very low repetition rate as indicated in FIG. 10. Thefirst packet of transmit signals with low acoustic amplitude are usedfor calibration. The echoes received from those signals are averaged toreduce the noise level.

The number of signals can be chosen such that a high signal-to-noiseratio is obtained. The second packet of signals with higher amplitudesare used to induce nonlinear propagation and harmonic distortion. Theechoes received from these signals are averaged and then the harmonicenergy is filtered and then compared to the calibration echo.

In order to estimate the volume of urine in the bladder, a look-up tablecan be created beforehand. Such a table, saved in the hard disk of theelectronic device, will contain the correspondence between the harmonicenergy and the volume of urine. Such a table can be extracted from acurve similar to the one given in FIG. 11. Such a curve can be obtainedfrom a “learning” patient set of measurements. Look-up tables mayeventually be produced for specific patient groups for age; genderand/or weight as an input parameter.

The described second method differs greatly from known other apparatus:

12) The device is composed of a single element defocused ultrasoundtransducer with a conical beam shape;

13) The single acoustic beam entirely encompasses the volumetric area ofa possibly filled bladder.

14) The method is based on measurement of non-linear properties andattenuation behavior of propagating ultrasound waves as influenced by aurine filled bladder.

15) The method incorporates a technique to eliminate patient variationdue to fat or skin properties.

16) The method for automatic volume computation does not assume anygeometrical model for the bladder shape;

17) It is valid for any bladder shape since the received signal“integrates” all volume effects in the ultrasound beam.

18) All known other methods use bladder wall echoes as a basis tocalculate volume.

19) The device works instantaneously. Other embodiments areintentionally within the scope of the accompanying claims.

We claim:
 1. An apparatus for measuring the volume of fluid in a humanor animal body cavity using a non-invasive, ultrasound echo technique,comprising: a transducer configured to: transmit at least a firstultrasound signal at a first power level into the body such that the atleast first ultrasound signal reaches a portion of the body cavity, thebody cavity being at least partially empty of the fluid such that therespective volumes of the body cavity and the fluid are different fromone another, and transmit at least a second ultrasound signal having afirst frequency and at a second power level higher than the first powerlevel such that the at least second ultrasound signal reaches theportion of the body cavity; a receiver component configured to receiveat least a first ultrasound echo signal from the body cavity for the atleast first ultrasound signal, and receive at least a second ultrasoundecho signal for the at least second ultrasound signal; and a processorcomponent configured to: determine a reference signal based on the atleast first ultrasound signal and the received at least first ultrasoundecho signal, wherein the reference signal is used to eliminate effectsof variation caused by patient anatomy, determine a measure of an energylevel of a harmonic component of the received at least second ultrasoundecho signal, wherein the harmonic component corresponds to at least asecond harmonic having at least two times the first frequency, re-scalethe reference signal based on an increase in power level between thefirst power level and the second power level, and determine the volumeof the fluid in the body cavity based on the measured energy level ofthe harmonic component and the re-scaled reference signal, whereindetermining the volume comprises accessing a table storingcorrespondences between harmonic energy levels and volumes of fluid. 2.The apparatus of claim 1, wherein the transducer comprises a curvedsingle active piezo-electric element, shaped to form a sector of asphere or cone like sound beam.
 3. The apparatus of claim 1, wherein thefirst frequency is a fundamental ultrasound frequency.
 4. The apparatusof claim 1, wherein the processor component is further configured to:determine, based on the determined volume, whether a threshold volumehas been reached.
 5. The apparatus of claim 4, wherein the processorcomponent is further configured to: provide an indication in response todetermining that the threshold volume has been reached.
 6. The apparatusof claim 1, wherein when transmitting at least a first ultrasoundsignal, the transducer is configured to: transmit a plurality of firstultrasound signals, the receiver component is configured to: receive aplurality of first echo signals corresponding to the plurality of firstultrasound signals, and the processor component is configured to:average the received plurality of first echo signals.
 7. The apparatusof claim 6, wherein when transmitting at least a second ultrasoundsignal, the transducer is configured to: transmit a plurality of secondultrasound signals, the receiver component is configured to: receive aplurality of second echo signals corresponding to the plurality ofsecond ultrasound signals, and the processor component is configured to:average the received plurality of second echo signals.
 8. The apparatusof claim 1, further comprising: a memory configured to store the table.9. A method for measuring the volume of fluid in a human or animal bodycavity using a non-invasive, ultrasound echo technique, comprising:using a transducer to: transmit at least a first ultrasound signal at afirst power level into the body such that the at least first ultrasoundsignal reaches a portion of the body cavity, the body cavity being atleast partially empty of the fluid such that the respective volumes ofthe body cavity and the fluid are different from one another; andtransmit at least a second ultrasound signal having a first frequencyand at a second power level higher than the first power level such thatthe at least second ultrasound signal reaches the portion of the bodycavity; receiving ultrasound echo signals from the body cavity for theat least first and the at least second ultrasound signals; determining areference signal based on the at least first ultrasound signal and thereceived at least first ultrasound echo signal; determining an energylevel of a harmonic component of the received at least second ultrasoundecho signal, wherein the harmonic component corresponds to at least asecond harmonic having at least two times the first frequency;re-scaling the reference signal based on an increase in power levelbetween the first power level and the second power level; anddetermining the volume of the fluid in the body cavity based on thedetermined energy level of the harmonic component and the re-scaledreference signal, wherein determining the volume comprises accessing atable storing correspondences between harmonic energy levels and volumesof fluid.
 10. The method of claim 9, further comprising: determining,based on the determined volume, whether a threshold volume has beenreached.
 11. The method of claim 10, further comprising: providing anindication in response to determining that the threshold volume has beenreached.
 12. The method of claim 9, wherein the transmitting at least asecond ultrasound signal comprises: transmitting a plurality of secondultrasound signals, and wherein the receiving ultrasound echo signalscomprises: receiving a plurality of second echo signals corresponding tothe plurality of second ultrasound signals, the method furthercomprising: averaging the received plurality of second echo signals. 13.The method of claim 12, wherein the transmitting at least a firstultrasound signal comprises: transmitting a plurality of firstultrasound signals, and wherein the receiving ultrasound echo signalscomprises: receiving a plurality of first echo signals corresponding tothe plurality of first ultrasound signals, the method furthercomprising: averaging the received plurality of first echo signals. 14.The method of claim 9, further comprising: generating and storing thetable in a memory.